[Adsorption Characteristics for Humic Acid by Binary Systems Containing Kaolinite and Goethite].

In this study, the binary systems of kaolinite-goethite mixture (KGM) and kaolinite-goethite complex (KGC) were prepared by different methods, and the surface properties and humic acid adsorption of the samples were investigated. Results showed that the specific surface area (SSA) of the samples followed the order of goethite> KGC> KGM> kaolinite, and the SSAs increased significantly for KGC while slightly for KGM when compared to the average value of kaolinite and goethite. The isoelectric point (IEP) of kaolinite, goethite, KGM and KGC appeared around 3.2, 7.9, 6.1 and 6.7, and the Zeta potential at pH 5.0 was -13.9, 38.2, 14.3 and 19.7 mV, respectively. The adsorption kinetic data for humic acid were well fitted using the pseudo-second-order kinetic models, suggesting that chemisorption was important in the adsorption process. Both one-site and two-site Langmuir models were suitable to describe the isotherm adsorption data (R² 0.962-0.993), and the correlation coefficients of two-site model for the binary systems were relatively higher (R2>0.989). The R2 values of Freundlich model fiting the adsorption data were low for the four samples, especially for the two pure samples. This indicated that the adsorption with various sites and mono-layer model was important in adsorbing humic acid onto the binary systems. At the initial pH of 5.0, the adsorption capacity (q) of kaolinite, goethite, KGM and KGC was 6.02, 61.83, 35.13 and 42.10 mg·g^-1, respectively. The q values of KGC and KGM increased to different extents when compared to the average of kaolinite and goethite. Thermodynamic parameters indicated that the adsorption of humic acid were endothermic for the four samples and non-spontaneous for kaolinite while spontaneous for the other samples.